In-vivo activation (red light) of hematoporphyrin derivative (Hpd), contained in malignant tumors, is curative in a wide range of animal and human tumors. We are investigating the fundamentals of this technique such as; mechanisms of accumulation of Hpd in malignant tissue, optimum wavelength of light for activation in-vitro and in-vivo, oxygen effect, tissue and cellular Hpd distribution and technology for light delivery using a laser (625-640nm) coupled to a fiber optic for treating remote and larger tumors.